let x1 = undefined
let x2 = null
let x3: symbol = Symbol('x3')

let x4: bigint = 100n
let x5: boolean = true
let x6: number = 100
let x7: string = 'hello'

const x8 = 'abc'

let x9: 'abc' = 'abc' // 字面类型

let x10 = new String('hello')

x10 = x8
x10 = x9
// primitive类型可以直接赋值给包装类型

let x11: string = 'xxxx'
x11 = x10.toString() // 包装类不能直接赋值给primitive类型，需要转换

class StringEx extends String {}
x10 = new StringEx('hello') // 子类可以直接赋值给父类
